The advent of the multimedia PC has been one of the key developments in the computer industry in the 1990s. Originally the term multimedia PC was loosely defined to refer to a personal computer with a CD-ROM and audio capabilities. Recently, however, new applications such as video conferencing, video-on-demand, interactive TV, and virtual reality have been proposed. Rather than the mere integration of text, audio and video, the nature of these applications require the transfer of high volumes of data between multiple users. As a result, it is now widely recognized that for multimedia to reach its full potential it must become a network based technology rather than a limited local resource.
Unfortunately, the real-time nature of multimedia video and audio streams renders existing local area networks ("LANs") unsuitable for these applications. Conventional LAN designs, most of which are based upon shared media architectures such as Ethernet and Token Ring, have no capability to guarantee the bandwidth and quality of service necessary to accommodate multimedia services. As such, these networks cannot efficiently handle high-speed, real-time video and audio data without introducing significant distortions such as delay, echo and lip synchronization problems.
Some network managers have responded to the increased demand for bandwidth by using routers and bridges to divide LANs into more segments, each with fewer users. In this manner, multimedia applications do not encounter significant competition for network resources from other forms of data traffic. However, such solutions alleviate congestion at the price of network complexity, and ultimately destroy a network's overall usefulness as a vehicle for sharing information.
A few modified forms of existing network technologies have been explored as platforms for multimedia transmissions. For instance, Isochronous Ethernet and Fiber Distributed Data Interface ("FDDI") can guarantee a minimum bandwidth connection to multimedia applications thereby providing the quality of service necessary to prevent distortions. However, such capability can only be provided under severe traffic constraints which are necessary to avoid unacceptable network performance degradation.
Recently, as the need for an alternative networking technology to accommodate multimedia in the LAN setting has become apparent, researchers have explored the technologies proposed for the Broadband Integrated Digital Services Network ("B-ISDN"). As high bandwidth requirements and bursty data transmission are commonplace in this wide area network, solutions used in B-ISDN may be applicable to the multimedia LAN environment.
Specifically, the B-ISDN standards, promulgated by the International Telegraph and Telephone Consultative Committee ("CCITT"), now reorganized as the Telecommunications Standardization Sector of the International Telecommunication Union ("ITU-T"), define a packet multiplexing and switching technique, referred to as Asynchronous Transfer Mode ("ATM"). This technique is well known in the art and is described in various references. E.g., Martin de Prycker, Asynchronous Transfer Mode: Solution for Broadband ISDN (2nd Ed., Ellis Horwood Ltd, West Sussex, England, 1993).
In ATM, information is carried in packets of fixed size, specified for B-ISDN as 53 bytes, called cells. Those cells are individually labelled by addressing information contained in the first 5 bytes of each cell. Cells from multiple sources are statistically multiplexed into a single transmission facility. Although ATM evolved from Time Division Multiplexing concepts, cells are identified by the contents of their headers rather than by their time position in the multiplexed stream. A single ATM transmission facility may carry hundreds of thousands of ATM cells per second originating from a multiplicity of sources and travelling to a multiplicity of destinations.
ATM is a connection-oriented technology. Rather than broadcasting cells onto a shared wire or fiber for all network members to receive, a specific routing path through the network, called a virtual circuit, is set up between two end nodes before any data is transmitted. Cells identified with a particular virtual circuit are only delivered to nodes on that virtual circuit and the cells transmitted on a particular virtual circuit are guaranteed to arrive in the transmitted order at the destination of the virtual circuit. ATM also defines virtual paths, bundles of virtual circuits traveling together through at least a portion of the network, the use of which can simplify network management.
The backbone of an ATM network includes switching devices capable of handling the high-speed ATM cell streams. These devices perform the functions required to implement a virtual circuit by receiving ATM cells from an input port, analyzing the information in the header of the incoming cells in real-time, and routing them to the appropriate destination port. Millions of cells per second need to be switched by a single switch.
Unlike conventional LAN designs, an ATM network makes no guarantees that it will deliver each and every packet to its intended destination. Rather, ATM provides the capability of offering multiple grades of service in support of various forms of traffic requiring different levels of cell loss probability and propagation delay. It is known, for instance, that many multimedia connections, e.g., video streams, can tolerate relatively large cell losses, but are very sensitive to delay variations from one cell to the next. In contrast, traditional forms of data traffic are more tolerant of large propagation delays and delay variance, but require very low cell losses. Because of its ability to handle diverse data streams, ATM appears to be an ideal platform for a LAN supporting both multimedia and conventional data transmission.
Despite the apparent applicability of ATM technology to the LAN environment, most existing ATM switches are not suitable for use in such an environment, for example, office or campus settings. Although a variety of problems exist, the principal barrier is cost: current switches, designed for use in large public networks cannot be cost-effectively installed in a local area network.
Much of the cost of current ATM switches can be attributed to expensive memory devices. Most conventional switches employ large First-In First-Out ("FIFO") memories at port inputs or outputs to queue cells awaiting transmission. Although this type of design is relatively simple, the high cost of such FIFO memories prices these switches out of the private market. Nor can these buffers simply be reduced in size: switches with reduced buffer sizes cannot achieve cell loss probabilities low enough to reliably handle the typical data traffic that LANs typically must accommodate.
Some switch designers have attempted to reduce the cost of large individual port buffer queues by replacing them with central shared buffer memories. These shared buffers may be smaller than the combined memory size of the individual port queues, because buffer space unused by a single port may be allocated to the other ports. However, the memory management schemes needed to maintain distinct cell queues within these shared buffers are quite complex. This complexity requires additional processing within the switch.
Nor is the cost attributable to memory devices limited to buffer queues. Additionally, most conventional switches employ large header translation look-up tables for each individual port. These tables must be large enough to support the large number of connections that a few ports of a switch may have to support, but typically most ports will have relatively few connections. Thus, table memory is wastefully reserved for these quiet ports.
Furthermore, despite their high-bandwidth performance, most current ATM switching systems have not been designed to accommodate the special requirements of many new multimedia applications. For instance, services such as video-on-demand or video conferencing between multiple locations require the capability to send the same cells to several destination users. Therefore, a "broadcast" or "multicast" facility within the switching system is essential to produce several copy cells from one received cell and to send those copy cells to the several destination systems. Although many conventional switching systems do provide some multicast capability, typically the capability has been added to switch architectures designed for switching cells from an input port to a single output port. These switches, which often must undertake a switching operation for each specified output port, cannot perform multicast cell switching without serious reduction in throughput performance. Processing of many multicast streams in a single switch can lead to severe network disturbance.
Additionally, because a wide variety of data traffic and network protocols can be expected in an ATM network integrated into a LAN environment, functions such as traffic shaping, policing and gateway translation may be required within the switches. However, currently the only switches possessing the flexibility necessary to handle these functions are full software switches which have the processing power to perform these tasks. Unfortunately, conventional software switches cannot achieve the cell throughput necessary to support the high-bandwidth requirements of multimedia streams.
Furthermore, switches designed for use in B-ISDN or other wide area networks are not easily adaptable to the different standards which will be applied in the LAN setting. Organizations such as the ATM Forum have already begun to produce ATM LAN standards which deviate in some significant ways from the CCITT standards. Furthermore, this standardization is by no means complete; current switches do not provide a mechanism for upgrading in the face of a constantly evolving standards environment.
Therefore, a need persists for an ATM switching device tailored to handle multimedia cell streams in a LAN environment, such as an office. Such a device would be inexpensive, easily upgradeable, and able to manage the special requirements of today's multimedia applications.